Power-Supply Circuit For An Electrical Appliance Having A Battery And A DC-TO-DC Converter

ABSTRACT

A power supply circuit for a small electrical appliance is disclosed. The power supply circuit includes a battery; a first load with a relatively high power consumption; a controllable switch; at least a second load with a low power consumption; and a DC-to-DC converter. The first load is connected to the battery via the controllable switch and the DC-to-DC controller supplies power from the battery to the second load, such that the controllable switch supplies to the first load a pulsed voltage having a pulse-pause ratio. The DC-to-DC converter powers the second load at least during the pauses.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of European Patent ConventionApplication No. 11007660.1, filed Sep. 16, 2011 and European PatentConvention Application No. 12004883.0, filed Jun. 29, 2012, thesubstance of which is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present disclosure is directed to a power-supply circuit for abattery-powered small electrical appliance which includes a first loadwith a relatively high power consumption, a controllable switch, atleast a second load with a relatively low power consumption and aDC-to-DC converter. The present disclosure is further directed to apower supply method for such a small electrical appliance.

BACKGROUND OF THE INVENTION

WO 02/15374 A1 describes a power supply circuit for electromotive smallelectrical appliances that are battery-powered and controlled by meansof a microcontroller. The power supply circuit comprises a DC-to-DCconverter that increases the voltage of the battery to a level that issufficient for the microcontroller. The DC-to-DC converter is a step-upconverter whose choke coil is formed by the electric motor. The electricmotor is controlled by the microcontroller with a pulse width modulatedvoltage in order to, on the one hand, enable the motor to run at thedesired speed and, on the other hand, enable the step-up converter toprovide sufficient voltage. However, the power supply circuit has thedisadvantage that the step-up converter does not supply voltage when thespeed of the electric motor is reduced—for instance when switching offthe motor.

EP 0 875 978 B1 describes a power supply circuit for electromotive smallelectrical appliances which are powered by an accumulator and controlledby means of a microcontroller. The power supply circuit comprises aDC-to-DC converter that increases the voltage of the accumulator to alevel that is sufficient for the microcontroller. If the accumulator issufficiently charged, the microcontroller is powered by the DC-to-DCconverter and the accumulator. If the accumulator is not sufficientlycharged, the microcontroller is powered via a capacitor while chargingthe accumulator by means of a charger, whereas the charging process ofthe accumulator is intermittently interrupted for short periods of timein order to recharge the capacitor.

Typical DC-to-DC converters contain an internal circuit that deactivatesthe DC-to-DC converter whenever the supply voltage drops below a minimumvoltage of e.g. 0.95 V. If the DC-to-DC converter powers a controlcircuit of a small electrical appliance containing a battery with onlyone cell (e.g. a NiMH battery with a nominal voltage of 1.2 V) and anelectric motor, the starting current (e.g. 7 A) of the electric motormight lead at least temporarily to a drop in battery voltage sufficientfor the DC-to-DC converter to deactivate and stop supplying power to thecontrol circuit, even with a fully-charged battery. This might occur inan old battery with a relatively high internal resistance (e.g. 50 mΩ).

There exists a need for a simple circuit and a method of power supply toa small electrical appliance as described above that better exploits anycharge still left in the battery. This need may be met by means of apower supply circuit in which the load with the relatively high powerconsumption is powered by a pulsed voltage having a pulse-pause ratio,and in which the DC-to-DC converter powers the load with the relativelylow power consumption at least during the pauses.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiment set forth in the drawing is illustrative in nature andnot intended to limit the invention defined by the claims The followingdetailed description of the illustrative embodiments can be understoodwhen read in conjunction with the following drawing, where likestructure is indicated with like reference numerals and in which:

FIG. 1 is a drawing of a power supply circuit according to oneembodiment.

SUMMARY OF THE INVENTION

In one embodiment, a power supply circuit for a small electricalappliance is provided. The power supply circuit includes a battery; afirst load with a relatively high power consumption; a controllableswitch; at least a second load with a low power consumption; and aDC-to-DC converter. The first load is connected to the battery via thecontrollable switch and the DC-to-DC controller supplies power from thebattery to the second load, such that the controllable switch suppliesto the first load a pulsed voltage having a pulse-pause ratio. TheDC-to-DC converter powers the second load at least during the pauses.

These and other features, aspects and advantages of specific embodimentswill become evident to those skilled in the art from a reading of thepresent disclosure.

DETAILED DESCRIPTION OF THE INVENTION

The following text sets forth a broad description of numerous differentembodiments of the present disclosure. The description is to beconstrued as exemplary only and does not describe every possibleembodiment since describing every possible embodiment would beimpractical, if not impossible. It will be understood that any feature,characteristic, component, composition, ingredient, product, step ormethodology described herein can be deleted, combined with orsubstituted for, in whole or part, any other feature, characteristic,component, composition, ingredient, product, step or methodologydescribed herein. Numerous alternative embodiments could be implemented,using either current technology or technology developed after the filingdate of this patent, which would still fall within the scope of theclaims. All publications and patents cited herein are incorporatedherein by reference.

According to the present disclosure, a power supply circuit is provided.In one embodiment, the power supply circuit has the advantage that asmall electrical appliance can be powered for a relatively long periodof time with one single battery charge when the battery presents a highinternal (e.g. age-related) resistance. As the load with the relativelyhigh power consumption, e.g. an electric motor, is powered by a pulsedvoltage and during the pauses the battery voltage exceeds the voltagepresent during the pulses, the DC-to-DC converter at least operatesduring the pauses and charges a capacitor connected in the usual mannerat its output terminal, the capacitor powering the load with therelatively low power consumption, for instance a control circuit, whichcontrols the functions of the small electrical appliance. In thismanner, the battery charge can be almost completely used—although at arelatively low voltage in older batteries. In one embodiment, the pulsedvoltage may have a pulse-pause ratio whose pauses are adjusted in such away that the DC-to-DC converter can adequately power the second load,even when only operating during the pauses.

In one embodiment, the DC-to-DC converter may exhibit a control port,with the DC-to-DC converter activated when the control port carriessufficient voltage. The control port is connected to the control circuitthat can switch the DC-to-DC converter on and off. The power supplycircuit may also contain a connection to a charger or a charging coil tocharge the accumulator, the former also being connected to the controlport of the DC-to-DC converter. The DC-to-DC converter is thereforeoperating as long as the charger or the charging coil suppliessufficient voltage, or as long as the control circuit keeps the DC-to-DCconverter operating. The control circuit can thus prevent the DC-to-DCconverter from automatically switching off in the usual manner duringperiods of low battery voltage. It is advantageous, for instance, forthe control circuit to ensure that the DC-to-DC converter remains inoperation during the short-term voltage drop that results from switchingon the motor. The DC-to-DC converter may still exhibit the usualinternal circuitry that switches off the DC-to-DC converter when thesupply voltage drops below a minimal voltage, whereby this minimalvoltage may be set to a relatively low value, which ensures that thebattery will not be deeply discharged by the DC-to-DC converter.

The power supply circuit may be suitable for a battery-powered smallelectrical appliance, such as an electric toothbrush or shaver, whosebatteries only consist of only one cell and whose battery voltages (e.g.1.2 V) must be increased by a DC-to-DC converter to a voltage (e.g. 3 V)suitable to the control circuit.

In one embodiment, the power supply circuit contains a charging coil L1connected to a battery B via a diode D1 and a first controllable switchS1, with one end of the charging coil L1 connected to the anode of thediode D1 and the other end of the charging coil L1 to the negative poleof the battery B (reference potential). A first capacitor C1 and aseries connection from an electric motor M and a second controllableswitch S2 are connected in parallel to the battery B. The battery B isalso connected to a DC-to-DC converter DC/DC. The DC-to-DC converterDC/DC further exhibits a control input and an output terminal, to whichone end of a second capacitor C3 is connected. The other end of thesecond capacitor C3 is connected to the negative pole of the battery B.A control circuit uC presents two terminals for its power supply, one ofwhich is connected to the output port of the DC-to-DC connector DC/DC,the other to the reference potential. The control circuit uC furtherexhibits an input which is connected to the positive pole of the batteryB as well as three outputs, one of which is connected to the firstcontrollable switch S1, the second to the second controllable switch S2,and the third to the control input of the DC-to-DC converter DC/DC andto one end of a resistor R4. The other end of the resistor R4 isconnected to the cathode of the diode D1, the first controllable switchS1 and one end of a second resistor R3, whose other end is connected tothe reference potential. The first controllable switch Si is onlyrepresented schematically. It may, for instance, consist of anelectronic circuit that will automatically activate when the chargingcoil applies sufficient voltage in order to charge the battery.

The method according to which this power supply circuit operates isexplained hereafter, initially assuming that the DC-to-DC converterDC/DC is not operating. As a consequence, the second capacitor C3 at theoutput port of the DC-to-DC converter is discharged, thus also renderingthe control circuit uC inoperative and leaving both the first Si and thesecond S2 controllable switch open. If a small electrical appliance withthis type of power supply circuit is to be switched on (if, forinstance, the motor M is to be put into operation), the small electricalappliance may be connected with a charger not represented in the FIGUREin order for the charging coil L1 to supply voltage via the diode D1 andthe resistor R4 to the control input of the DC-to-DC converter. If thevoltage at the control input is sufficiently high, the DC-to-DCconverter switches on and charges the second capacitor C3 connected toits output (e.g. to a voltage of 3 V). However, the DC-to-DC converterrequires a minimum voltage in order to supply power and therefore,despite the sufficiently high voltage at its control input, will notsupply power if the voltage at the battery (e.g. a NiMH accumulator witha nominal voltage of 1.2 V) or at the first capacitor C1 is below theminimum voltage.

If the voltage at the second capacitor C3 is sufficiently high, thecontrol circuit uC switches on and takes control of the small electricalappliance including the motor M (via the controllable switch S2), theDC-to-DC converter DC/DC and the battery charging process (in particularthe termination of the charging process by opening the controllableswitch S1 when the battery reaches full charge condition) as well asadditional loads, where required, not described in the FIGURE, such as adisplay. The control of the small electrical appliance depends on thebattery voltage or the voltage at the first capacitor C1, which iscontinuously monitored by the control circuit. It is particularlyadvantageous if the control circuit keeps the DC-to-DC converter inoperating condition during the start-up of the motor by applying arespective signal to the control input of the DC-to-DC converter. Shouldthe voltage at the first capacitor C1 temporarily drop below the minimumvoltage of the DC-to-DC converter, due to the signal applied to thecontrol input the DC-to-DC converter will immediately resume powersupplying as soon as the minimum voltage is reached again.

In one embodiment, the control circuit uC powers the motor M with apulsed voltage, which may exhibit a fixed frequency (fixed cycleduration) and a fixed pulse-pause ratio. The control circuit controlsthe second controllable switch S2, for example, in such a way that itwill be switched on during 95-99% of the cycle and is only inactiveduring 1-5% of the cycle. In one embodiment, the control circuit may beoperational at a frequency of about 250 Hz and is switched on duringapproximately 98% of the cycle time, while the motor M is running Thelength of the pauses of the pulsed voltage, the properties of theDC-to-DC converter, the size of the second capacitor C3 (e.g. 10 μF),and the power consumption of the control circuit uC (e.g. TI MSP 430)are optimally coordinated in order for the control circuit to receive asufficient power supply, even when the DC-to-DC converter is chargingthe second capacitor C3 only during the pauses. This might occur whenthe battery voltage is relatively low, that is, the battery is almostempty, and/or the motor M has a particularly high power requirement—forinstance during start-up. If the battery voltage is relatively high, theDC-to-DC converter may be operating during the whole cycle length andsupplying sufficient current to not only power the control circuit uC,but the other loads as well. With medium battery voltage and a runningmotor M, the situation can arise that the voltage at the first capacitorC1 drops below the minimum voltage of the DC-to-DC converter after halfa cycle and thus will stop supplying power. The control circuit willthen temporarily switch off the other loads and will draw power from thesecond capacitor C3 during the remaining cycle time, while the DC-to-DCconverter will recharge the second capacitor C3 during the pause (if theDC-to-DC converter's minimum voltage has been reached again afterswitching off of the motor).

The above-described power supply circuit of a small electrical appliancehence operates according to a method wherein the load with a relativelyhigh power consumption is powered by a pulsed voltage having apulse-pause ratio and the load with a relatively low power consumptionis powered by the DC-to-DC converter, at least during the pauses. In oneembodiment, the pauses may be adjusted such that the load with therelatively low power consumption will receive sufficient power even withthe DC-to-DC converter merely operating during the pauses. If thebattery voltage is higher than a determined minimum voltage, theDC-to-DC converter will power the second and further loads even duringthe pulses. The control circuit furthermore provides continuousmonitoring of the battery voltage and switches the small electricalappliance off whenever necessary. If, for instance, the battery voltageis lower than a first threshold value directly after switching off themotor, or the battery voltage is lower than a second threshold valueduring operation of the motor, the control circuit may first disconnectthe motor, then the other loads, and lastly the DC-to-DC converter. Assoon as the voltage at the second capacitor C3 has dropped accordingly,the control circuit uC will also disconnect. In this manner, a deepdischarge of the battery can be avoided. Then the small electricalappliance can only be put back into operation by charging the battery B.

In one embodiment, the control circuit uC may be implemented in amicrocontroller.

The dimensions and values disclosed herein are not to be understood asbeing strictly limited to the exact numerical values recited. Instead,unless otherwise specified, each such dimension is intended to mean boththe recited value and a functionally equivalent range surrounding thatvalue. For example, a dimension disclosed as “40 mm” is intended to mean“about 40 mm.”

Every document cited herein, including any cross referenced or relatedpatent or application, is hereby incorporated herein by reference in itsentirety unless expressly excluded or otherwise limited. The citation ofany document is not an admission that it is prior art with respect toany invention disclosed or claimed herein or that it alone, or in anycombination with any other reference or references, teaches, suggests ordiscloses any such invention. Further, to the extent that any meaning ordefinition of a term in this document conflicts with any meaning ordefinition of the same term in a document incorporated by reference, themeaning or definition assigned to that term in this document shallgovern.

While particular embodiments of the present invention have beenillustrated and described, it would be obvious to those skilled in theart that various other changes and modifications can be made withoutdeparting from the spirit and scope of the invention. It is thereforeintended to cover in the appended claims all such changes andmodifications that are within the scope of this invention.

What is claimed is:
 1. A power supply circuit for a small electricalappliance, comprising: a battery; a first load with a relatively highpower consumption; a controllable switch; at least a second load with alow power consumption; and a DC-to-DC converter; wherein the first loadis connected to the battery via the controllable switch and the DC-to-DCcontroller supplies power from the battery to the second load, such thatthe controllable switch supplies to the first load a pulsed voltagehaving a pulse-pause ratio, and wherein the DC-to-DC converter powersthe second load at least during the pauses.
 2. A power supply circuitaccording to claim 1, wherein the first load is an electric motor andthe second load is a control circuit.
 3. A power supply circuitaccording to claim 1, wherein the pulsed voltage has a pulse-pause ratiowhose pause is adjusted in such a way that the DC-to-DC converter cansufficiently power the second load even when only operating during thepauses.
 4. A power supply circuit according to claim 1, wherein theDC-to-DC converter exhibits a control input and can be switched on andoff by applying the appropriate voltages to the control input.
 5. Apower supply circuit according to claim 4, further comprising a chargingcoil, and wherein the DC-to-DC converter can be activated by thecharging coil by applying a voltage to the control input.
 6. A powersupply circuit according to claim 4, wherein the control input of theDC-to-DC converter is connected to the control circuit.
 7. A smallelectrical appliance with a power supply circuit according to claim 1.